<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .container {
            width: 100px;
        }

        .avatar {
            padding: 50%;
            background-color: yellow;
            border-radius: 50%;
        }
    </style>
</head>

<body>
    <div class="container">
        <div class="avatar"></div>
    </div>
</body>

</html>